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1. 1THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Chapter 13
Reliability

2. 2THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Reliability
• Generally defined as the ability of a
product to perform as expected over time
• Formally defined as the probability that a
product, piece of equipment, or system
performs its intended function for a stated
period of time under specified operating
conditions

3. 3THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Maintainability
• The probability that a system or product
can be retained in, or one that has failed
can be restored to, operating condition in
a specified amount of time.

4. 4THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Types of Failures
• Functional failure – failure that occurs
at the start of product life due to
manufacturing or material detects
• Reliability failure – failure after some
period of use

5. 5THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Types of Reliability
• Inherent reliability – predicted by
product design
• Achieved reliability – observed during
use

6. 6THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Reliability Measurement
• Failure rate (λ) – number of failures
per unit time
• Alternative measures
– Mean time to failure
– Mean time between failures

7. 7THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Cumulative Failure Rate Curve

8. 8THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Failure Rate Curve
“Infant
mortality
period”

9. 9THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Average Failure Rate

10. 10THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Reliability Function
• Probability density function of failures
f(t) = λe-λt
for t > 0
• Probability of failure from (0, T)
F(t) = 1 – e-λT
• Reliability function
R(T) = 1 – F(T) = e-λT

11. 11THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Series Systems
RS = R1 R2 ... Rn
1 2 n

12. 12THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Parallel Systems
RS = 1 - (1 - R1) (1 - R2)... (1 - Rn)
1
2
n

13. 13THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Series-Parallel Systems
• Convert to equivalent series system
AA BB
CC
CC
DD
RRAA RRBB
RRCC
RRDD
RRCC
AA BB C’C’ DD
RRAA RRBB RRDD
RRC’C’ = 1 – (1-R= 1 – (1-RCC)(1-R)(1-RCC))

14. 14THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Reliability Engineering
• Standardization
• Redundancy
• Physics of failure
• Reliability testing
• Burn-in
• Failure mode and effects analysis
• Fault tree analysis

15. 15THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM

16. 16THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM

17. 17THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Reliability Management
• Define customer performance requirements
• Determine important economic factors and
relationship with reliability requirements
• Define the environment and conditions of product
use
• Select components, designs, and vendors that meet
reliability and cost criteria
• Determine reliability requirements for machines
and equipment
• Analyze field reliability for improvement

18. 18THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Configuration Management
• Establish approved baseline
configurations (designs)
• Maintain control over all changes in the
baseline programs (change control)
• Provide traceability of baselines and
changes (configuration accounting)

19. 19THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Maintainability
• Maintainability is the totality of design
factors that allows maintenance to be
accomplished easily
• Preventive maintenance reduces the risk
of failure
• Corrective maintenance is the response to
failures

20. 20THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Design Issues
• Access of parts for repair
• Modular construction and
standardization
• Diagnostic repair procedures and
expert systems

21. 21THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM
Availability
• Operational availability
• Inherent availability
MDTMTBM
MTBF
AO
+
=
MTTRMTBF
MTBF
AO
+
=
MTBM = mean time between
maintenance
MTD = mean down time
MTBF = mean time between
failures
MTTR = mean time to repair